Aluminum-based materials and aluminum-containing composites are known to be relatively resistant to oxidation corrosion. However, such materials are susceptible to pitting corrosion when exposed to acids and halides. For example, aluminums deteriorate rapidly when exposed to sea water. Even aluminum alloys which are only exposed to the atmosphere will deteriorate with time because of pitting corrosion caused by acidic air pollutants and acid rain.
Traditionally, there have been two commonly used methods of increasing the pitting corrosion resistance of aluminum alloys: anodizing and passivation with chromate solutions. Neither of these methods, however, is wholly satisfactory. Anodizing involves a complex and expensive multi-step procedure. Chromate passivation involves a less complex procedure, but does not provide long-term corrosion protection. Chromate passivation, for example, does not provide sufficient pitting corrosion protection to allow aluminum-based materials to be used in marine environments.
Two of us (Mansfeld and Wang) have recently participated in the development of a third method of increasing the pitting corrosion-resistance of aluminums. This third method is described in U.S. Pat. No. 5,194,138 which is incorporated herein by reference in its entirety. In this third method, aluminum-based materials are first contacted with an aqueous cerium non-halide solution and thereafter contacted with a cerium halide solution. This new process is simpler and less expensive than anodizing methods. It also yields superior results to results obtained from chromate pacification methods.
Unfortunately, neither anodizing, chromate pacification, nor even the new cerium solution treatment works particularly well on aluminum-copper alloys wherein the copper content of the alloy is more than about one percent. These alloys, such as Aluminum Associate alloy types Al 7075 (1.2-2.0% copper) and Al 2024 (3.8-4.9% copper), have become increasingly popular because they possess certain mechanical properties which are superior to other aluminum alloys. Unfortunately, however, they cannot be made substantially resistant to localized corrosion by any known method.
Accordingly, there is a need for a method of increasing the corrosion resistance of aluminum alloys wherein the copper content is greater than about one percent.